We propose to use dissociated cell culture techniques to study the differentiation of the membrane properties of rat spinal cord neurons and heart muscle cells. Intracellular recording and voltage clamp methods will be used to measure the voltage dependent membrane currents at various stages in the development of these cells. The different membrane currents will be separated from each other by use of specific blocking agents (e.g., tetrodotoxin to block the fast sodium-dependent channel) and by altering the ionic composition of the medium bathing the cells. We will test the effects of media conditioned by various cell types in an attempt to isolate "trophic factors" which might affect the differentiation of the neuron membrane properties. We will also test for trophic effects of this conditioned medium on axonal growth and transmitter synthesis by the neurons. specifically we will assay cultures treated with fibroblastor muscle cell-conditioned medium for effects on the amount of choline acetyltransferase activity in the spinal cord neurons.